Plating film and forming method thereof

ABSTRACT

Tin plating film composed of tin or tin alloy is formed on a front surface and a rear surface of a substrate composing a lead frame. As tin alloy, for example, tin-copper alloy (content of copper: 2 mass %), tin-bismuth alloy (content of bismuth: 2 mass %) and the like can be cited. The substrate is composed of, for example, Cu alloy or the like. Within the tin plating film, plural crystal grains are arranged irregularly. Further, plural gap portions exist within the tin plating film. An external stress is reduced even if a bending process or the like are performed subsequently, because the gap portions exist within the tin plating film. Consequently, growths of whiskers accompanied by the external stress are suppressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 11/475,169,filed on Jun. 27, 2006 which is based upon and claims the benefit ofpriority from the prior Japanese Patent Application No. 2006-083074,filed on Mar. 24, 2006, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a plating film and a forming methodthereof suitable for a terminal of a semiconductor chip and a terminalof a connector.

2. Description of the Related Art

Tin-lead solder plating has been performed for a terminal for aconnector, a lead frame for a semiconductor integrated circuit and soon. However, in recent years, utilizations of tin plating, tin-copperalloy plating, tin-bismuth alloy plating, tin-silver alloy plating, andso on which do not contain lead are examined instead of the tin-leadsolder plating from a point of view of an environmental conservation.For example, an art performing the tin-copper alloy plating is disclosedin Patent Document 1 (Japanese Patent Application Laid-open No.2001-26898).

However, when a membrane composed of the above-stated alloy notcontaining lead is formed, whisker-state crystals of tin called aswhiskers become easy to be generated during use and so on. An electricalshort circuit failure may occur between adjacent electrodes with eachother if the whiskers are generated and grow. Besides, a diameter of thewhisker is narrow, approximately 1 μm, a length thereof may reach 1000μm or more. Consequently, the whiskers may be detached from the membraneto be scattered. If the whiskers are scattered, the whiskers may causethe short circuit failure inside/outside of a device.

Incidentally, an internal stress and an external stress of a platingfilm can be cited as one of a generation cause of the whiskers. As theinternal stress, a stress due to a mismatching of a lattice constantwith a base metal, a growth of an intermetallic compound caused by adiffusion reaction between the base metal (for example, Cu atom) and Sn,a gloss agent inside of the plating or the like can be cited. On theother hand, as the external stress, a stress received at the time of abending process and a punching process performed after the plating orthe like can be cited in the lead frame, and a stress received when acontact is engaged in the terminal or the like can be cited for theconnector.

The internal stress can be reduced by performing mat plating orsemi-gloss plating by using a plating solution in which a glosscomponent is extremely decreased. Besides, it is confirmed that thegeneration of the whiskers is suppressed by reducing the stress byperforming a heat treatment at approximately 150° C. after the plating.Furthermore, it is also effective that a diffusion barrier layercomposed of nickel or the like is plated to the base metal in advance tosuppress the growth of the intermetallic compound.

As stated above, methods to suppress the growth of the whiskersaccompanied by the internal stress exists, but a method to suppress thegrowth of the whiskers accompanied by the external stress is not known.Consequently, it cannot be said that a suppression of the short circuitfailure and so on are sufficient because the growth of the whiskeraccompanied by the external stress cannot be suppressed even though theinternal stress can be suppressed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a plating film and aforming method thereof capable of effectively suppressing a growth ofwhiskers accompanied by an external stress.

The present inventor comes to an idea of various embodiments of theinvention as shown in the following as a result of hard study to solvethe above-stated problems.

A plating film according to the present invention is a plating filmcomposed of tin or tin alloy formed on a surface of a substrate, whereingap portions exist between crystal grains in the film.

In a forming method of a plating film according to the presentinvention, a substrate is immersed in a plating solution, and thereafterelectrolysis of the plating solution is performed while making thesubstrate a cathode. At that time, a concentration of a surfactant to beadded to the plating solution is set as 10 g/liter or less, and acurrent flowing in the cathode is set as 2.5 A/dm² or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a lead frame including a plating film accordingto an embodiment of the present invention;

FIG. 2 is a sectional view showing the lead frame;

FIG. 3 is a view showing an appearance of a PGA package;

FIG. 4 is a view showing an appearance of an SOP;

FIG. 5A is a view showing an appearance of a male connector;

FIG. 5B is a view showing an appearance of a female connector; and

FIG. 6 is a view showing an appearance of a USB connector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are concretelydescribed with reference to the attached drawings. FIG. 1 is a viewshowing a lead frame including a plating film according to an embodimentof the present invention. Besides, FIG. 2 is a sectional view showing across-sectional structure of the lead frame.

A dye pad 1 on which a semiconductor chip is to be mounted is providedat a lead frame 10, plural holes 4 a extending radially are formed incircumference thereof. Inner lead portions 2 are formed between adjacentholes 4 a. Besides, plural holes 4 b are formed at outside of the holes4 a, and outer lead portions 3 are formed between adjacent holes 4 b.

Besides, as shown in FIG. 2, a tin plating film 13 composed of tin ortin alloy is formed on a front surface and a rear surface of a substrate11 composing the lead frame 10. As tin alloy, for example, tin-copperalloy (content of copper: 2 mass %), tin-bismuth alloy (content ofbismuth: 2 mass %) and the like can be cited. The substrate 11 iscomposed of, for example, Cu alloy or the like. Within the tin platingfilm 13, plural crystal grains 12 are arranged irregularly. Further, inthe present embodiment, plural gap portions 14 exist within the tinplating film 13. Incidentally, in FIG. 2, a shape of each crystal grain12 is an ellipse, but an actual shape thereof is a polygon, the one acurve is combined with a polygon, or the like.

As to the lead frame 10 constituted as stated above, bonding betweenterminals of the semiconductor chip and the inner lead portions 2 areperformed after the semiconductor chip is fixed to the dye pad 1.Subsequently, portions between the holes 4 a and the holes 4 b are cut.A bending process of the outer lead portions 3 and the like areperformed after sealing of the semiconductor chip and the inner leadportions 2 are performed. As a result, a PGA package (Pin Grid Array) orthe like can be obtained. In FIG. 3, an appearance of the PGA package isshown. In the PGA package, lead terminals 22 are extending in the samedirection with each other from a housing 21 composed of ceramic or thelike. Besides, an SOP (Small Out-line Package) as shown in FIG. 4 or thelike can also be obtained, if a pattern of the lead frame is changed. Inthe SOP, plural lead terminals 32 are extending from two long edges of ahousing 31 of which planer shape is rectangle.

When a conventional plating film is formed, whiskers are easy to begenerated by an external stress operated at the time of the bendingprocess. On the contrary, in the present embodiment, the external stressis reduced because the gap portions 14 exist within the tin plating film13, even if the bending process or the like is performed. Consequently,the growths of the whiskers accompanied by the external stress aresuppressed.

Incidentally, a maximum diameter of the gap portion 14 is preferable tobe 50% or less of a thickness of the tin plating film 13. This isbecause damages may occur to the tin plating film 13 resulting from aninsufficiency of a mechanical resistance for the external stress if themaximum diameter of the gap portion 14 exceeds the thickness of the tinplating film 13.

Besides, from a point of view of the mechanical resistance, if ahardness of the plating film is defined, the hardness of the platingfilm measured by a nanoindentation method is preferable to be 150 MPa to400 MPa. If the hardness of the plating film is less than 150 MPa, themechanical resistance may become insufficient, and if the hardness ofthe plating film is over 400 MPa, it becomes easy to be affected by theexternal stress.

Besides, a ratio of the gap portion 14 within the tin plating film 13 ispreferable to be 5 vol % to 30 vol %. If the ratio of the gap portion 14is less than 5 vol %, an effect to reduce the external stress may beeliminated, and if the ratio of the gap portion 14 is over 30 vol %, theresistance may become insufficient.

In an electroplating method to form the tin plating film containing gapportions as stated above, an amount of a surfactant added to a platingsolution is set as 10 g/liter or less and a current density is set as2.5 A/dm² or more, for example. If the surfactant is added more than 10g/liter, the tin plating film to be formed becomes precise, and the gapportions are easy to go short. Besides, when the current density is setas less than 2.5 A/dm², a crystal grain diameter composing the tinplating film becomes small and the gap portions are easy to go short. Inthe electroplating method, the substrate (material to be plated) is madeto be a cathode to perform electrolysis of the plating solution.Incidentally, as the surfactant, for example, a nonionic surfactant, acationic surfactant, an anionic surfactant, an amphoteric surfactant andthe like can be used. Examples of usable surfactants are cited in thefollowing. However, it is not limited to these.

As the nonionic surfactant, polyoxyalkylene alkylether, polyoxyalkylenenaphthylether, polyoxyalkylene bisphenol ether, polyoxyethylenepolyoxypropylene block polymer, polyoxyalkylene sorbitan fatty acidester, polyoxyalkylene glycerin fatty acid ester, polyoxyalkylenealkylamine, polyoxyalkylene alkyl phenyl formalin condensate,oxyethylene alkylamine and the like can be cited.

As the cationic surfactant, alkyl trimethyl ammonium halide,hydroxyethyl alkyl imidazoline, dialkyl dimethyl ammonium halide, alkyldimethyl benzyl ammonium halide, alkyl amine hydrochloride, alkyl amineacetate, alkyl amine oleate, alkyl aminoethyl glycine and the like canbe cited.

As the anionic surfactant, fatty acid soap series surfactant, alkylsulfonate, alpha olefin sulfonate, alkyl diphenyl ether disulfonate,polyoxyethylene alkylether sulfuric ester salt, higher alcoholphosphoric monoester salt, polyoxyalkylene alkylether phosphoric acid(polyoxyalkylene alkylether phosphate), polyoxyalkylene phenyl etherphosphate, polyoxyethylene alkylether acetate, alkanoyl methylalaninesalt, N-acyl sulfocarboxylate, alkyl sulfoacetate, sulfosuccinicmonooleylamide salt and the like can be cited.

As the amphoteric surfactant, 2-alkyl-N-carboxymethyl (orethyl)-N-carboxymethyloxyethyl imidazolynium betaine, dimethyl alkylbetaine, N-alkyl-beta-aminopropionic acid (or salt thereof),alkyl(poly)aminoethyl glycine and the like can be cited.

As tin salt or tin complex used for the tin plating process, forexample, salt of inorganic acid such as tin sulfate, tin fluoroborate,tin hydrofluorosilicic acid, tin sulfamate, tin stannate, tinpyrophosphate tin and the like can be used. Besides, aliphatic sulfonatesuch as tin methanesulfonate, tin sulfosuccinate and the like may beused. Further, compound salt and the like having carboxyl group such astin succinate, tin malonate, tin gluconate and the like may be used.

Besides, one kind or two or more kinds from a smoothing agent, a glossagent, a pH buffering agent, and/or conductive salt may be added to theplating solution.

Incidentally, it is preferable to perform a cathode electrolyticdegreasing process and a chemical polishing of a substrate (material tobe plated) before the plating process. Furthermore, it is preferable toperform a washing of the substrate after the cathode electrolyticdegreasing process and the chemical polishing. As an electrolyticdegreasing agent, for example, Cleaner 160 manufactured by Meltex co.Ltd. can be used. As a polishing agent, for example, 50% CPB40manufactured by Mitsubishi Gas chemical company Inc. can be used.

The gap portion is not necessarily be a complete cavity but fineparticles such as resin particles or ceramic particles may exist withinthe gap portion. For example, resin powders or ceramic powders are addedto the plating solution, and these particles are to be precipitatedsimultaneously with a forming of the plating film to form such tinplating film. Discontinuous grain boundaries are formed within the tinplating film by dispersing these particles within the tin plating film,and the external stress is reduced. Other particles than the resinparticles or the ceramic particles may be used if the external stresscan be reduced as stated above.

Incidentally, in the above-stated embodiment, the tin plating film isformed on the surface of the lead frame, but an object on which the tinplating film is to be formed is not limited to the lead frame. Forexample, it may be formed on a surface of a terminal of a connector. Asthe connector, for example, a male connector 41 shown in FIG. 5A, afemale connector 51 shown in FIG. 5B, an USB (Universal Serial Bus)connector 61 shown in FIG. 6 and the like can be cited.

The forming of the tin plating film to the connector is performed after,for example, a bending process and the like are performed, andtherefore, the external stress may not operate on the tin plating filmat the time of the bending process, but the external stress may operateto the tin plating film when the connector is engaged with a destinationfor connection. The whiskers become difficult to be generated ifappropriate gap portions as same as the above-described embodiment areformed even in such a case.

Incidentally, a thickness of a tin plating film formed on a terminal ofa connector is approximately 3 μm in general. This is because a goodengagement can be realized with the thickness as stated above. If thethickness of the tin plating film is approximately 3 μm, a maximumdiameter of the gap portion is preferable to be approximately 1.5 μm,which is half of the maximum diameter of the tin plating film, as sameas the case of the lead frame. Besides, the thickness of the tin platingfilm is preferable to be 2 μm to 3 μm regardless for which terminal itis used. If the thickness is less than 2 μm, there is a case when asufficient function as a plating film (protection of a substrate and soon) can not be exerted. If the thickness is over 3 μm, variation of thethickness is easy to occur. Besides, as a plating base material, thereis not a restriction in particular, and a 42 alloy material, a brassmaterial, a phosphor bronze material, a beryllium copper material, acopper material, a nickel material and the like can be used.Furthermore, the one constituted by forming a nickel plating film, acopper plating film or the like on the surface of a substrate can beused.

Next, an examination actually performed by the present inventor isdescribed. In this examination, a lead terminal with 40 pins made ofphosphorus bronze was used as a substrate.

First, a cathode electrolytic degreasing process was performed to asubstrate as a pre-process of plating. Cleaner 160 manufactured byMeltex co. Ltd. was used as an electrolytic degreasing agent. In thedegreasing process, a process temperature was set at 65° C., a currentdensity was set at 2.5 A/dm², a processing time was set for 30 seconds.After the cathode electrolytic degreasing process was completed, thesubstrate was washed.

Next, the substrate is chemically polished. 50% CPB40 manufactured byMitsubishi Gas chemical company Inc. was used as a polishing agent. Inthe chemical polishing, a temperature of the agent was set to beapproximately at a room temperature, and an immersing time was set for20 seconds. After the chemical polishing was completed, the substratewas washed.

Subsequently, a plating process was performed for the substrate, tothereby form a tin plating film. In the plating process, the processtemperature was set at 30° C., and the processing time was set for 20seconds. Besides, the thickness of the tin plating film wasapproximately 3 μm. A composition of a plating process solution used inthe plating process is shown in Table 1. Incidentally, methanol withinthe plating process solution was contained to disperse tin methanesulfonate, methane sulfonic acid, and polyoxyalkylene bisphenol etheruniformly.

TABLE 1 Comparative Example Example Example Example Example Exampleexample 1 2 3 4 5 6 Tin 17 17 17 17 17 17 17 Methane- sulfonate (g/l)Methane- 36 46 46 51 51 54 54 sulfonic acid (g/l) Polyoxy- 20 10 10 5 52 2 alkylene bisphenol ether (g/l) Methanol 27 27 27 27 27 27 27 (g/l)

An evaluation of the whiskers was performed as for connector leadterminals on which the tin plating films were formed. In the evaluation,the male connector and the female connector were engaged with each otherand then left for 2000 hours in room temperature. Subsequently, surfacesof respective samples were observed with a microscope using amagnification of 100 power. If a whiskers was found, a detailedobservation was performed by using the microscope with higher-power. Theresult was shown in Table 2.

TABLE 2 Comparative Example Example Example Example Example Exampleexample 1 2 3 4 5 6 Current 10 10 15 15 20 20 25 Density (A/dm²) GapRatio 0 2 5 8 19 30 38 (%) Hardness 500 320 250 200 180 40 30 (MPa)Whiskers >50 3 0 0 0 0 0 generation number

As shown in Table 2, in the comparative example, in which the gapportions did not exist within the tin plating film, the whiskers weregenerated in bulk. The whiskers were also generated in the example 1because the ratio of the gap portion was small, but the number was verysmall. On the other hand, in the example 6, in which the ratio of thegap portion was over 30%, the generation of the whiskers could beprevented completely, but the plating hardness was low. It can also besaid that the ratio of the gap portion is preferable to be 5% to 30%from these results.

According to the present invention, the external stress is reduced bythe gap portions even if the external stress operates on the platingfilm. Consequently, a growth of the whiskers accompanied by the externalstress can be suppressed.

1. A forming method of a plating film composed of tin or tin alloy,comprising: immersing a substrate in a plating solution, a concentrationof a surfactant to be added to said plating solution being set as 10g/liter or less; and performing electrolysis of said plating solutionwhile making said substrate a cathode, a current flowing in said cathodebeing set as 2.5 A/dm² or more.